Molten bath smelting or other pyrometallurgical operations which require interaction between the bath and a source of oxygen-containing gas utilize several different arrangements for the supply of the gas. In general, these operations involve direct injection into molten matte/metal. This may be by bottom blowing tuyeres as in a Bessemer type of furnace or side blowing tuyeres as in a Peirce-Smith type of converter. Alternatively, the injection of gas may be by means of a lance to provide either top blowing or submerged injection. Examples of top blowing lance injection are the KALDO and BOP steel making plants in which pure oxygen is blown from above the bath to produce steel from molten iron. Another example of top Mitsubishi copper process, in which injection lances cause jets of oxygen-containing blowing lance injection is provided by the smelting and matte converting stages of the gas such as air or oxygen-enriched air, to impinge on and penetrate the top surface of the bath, respectively to produce and to convert copper matte. In the case of submerged lance injection, the lower end of the lance is submerged so that injection occurs within rather than from above a slag layer of the bath, to provide top submerged lancing (TSL) injection, a well known example of which is the Outotec Ausmelt TSL technology which is applied to a wide range of metals processing.
With both forms of injection from above, that is, with both top blowing and TSL injection, the lance is subjected to intense prevailing bath temperatures. The top blowing in the Mitsubishi copper process uses a number of relatively small steel lances which have an inner pipe of about 50 mm diameter and an outer pipe of about 100 mm diameter. The inner pipe terminates at about the level of the furnace roof, well above the reaction zone. The outer pipe, which is rotatable to prevent it sticking to a water-cooled collar at the furnace roof, extends down into the gas space of the furnace to position its lower end about 500-800 mm above the upper surface of the molten bath. Particulate feed entrained in air is blown through the inner pipe, while oxygen enriched air is blown through the annulus between the pipes. Despite the spacing of the lower end of the outer pipe above the bath surface, and any cooling of the lance by the gases passing through it, the outer pipe burns back by about 400 mm per day. The outer pipe therefore is slowly lowered and, when required, new sections are attached to the top of the outer, consumable pipe.
The lances for TSL injection are much larger than those for top blowing, such as in the Mitsubishi process described above. A TSL lance usually has at least an inner and an outer pipe, as assumed in the following, but may have at least one other pipe concentric with the inner and outer pipes. Typical large scale TSL lances have an outer pipe diameter of 200 to 500 mm, or larger. Also, the lance is much longer and extends down through the roof of a TSL reactor, which may be about 10 to 15 m tall, so that the lower end of the outer pipe is immersed to a depth of about 300 mm or more in a molten slag phase of the bath, but is protected by a coating of solidified slag formed and maintained on the outer surface of the outer pipe by the cooling action of the injected gas flow within. The inner pipe may terminate at about the same level as the outer pipe, or at a higher level of up to about 1000 mm above the lower end of the outer pipe. Thus, it can be the case that the lower end of only the outer pipe is submerged. In any event, a helical vane or other flow shaping device may be mounted on the outer surface of the inner pipe to span the annular space between the inner and outer pipes. The vanes impart a strong swirling action to an air or oxygen-enriched blast along that annulus and serve to enhance the cooling effect as well as ensure that gas is mixed well with fuel and feed material supplied through the inner pipe with the mixing occurring substantially in a mixing chamber defined by the outer pipe, below the lower end of the inner pipe where the inner pipe terminates a sufficient distance above the lower end of the outer pipe.
The outer pipe of the TSL lance wears and burns back at its lower end, but at a rate that is considerably reduced by the protective frozen slag coating than would be the case without the coating. However, this is controlled to a substantial degree by the mode of operation with TSL technology. The mode of operation makes the technology viable despite the lower end of the lance being submerged in the highly reactive and corrosive environment of the molten slag bath. The inner pipe of a TSL lance may be used to supply feed materials, such as concentrate, fluxes and reductant to be injected into a slag layer of the bath, or it may be used for fuel. An oxygen containing gas, such as air or oxygen enriched air, is supplied through the annulus between the pipes. Prior to submerged injection within the slag layer of the bath being commenced, the lance is positioned with its lower end, that is, the lower end of the outer pipe, spaced a suitable distance above the slag surface. Oxygen-containing gas and fuel, such as fuel oil, fine coal or hydrocarbon gas, are supplied to the lance and a resultant oxygen/fuel mixture is fired to generate a flame jet which impinges onto the slag. This causes the slag to splash to form, on the outer lance pipe, the slag layer which is solidified by the gas stream passing through the lance to provide the solid slag coating mentioned above. The lance then is able to be lowered to achieve injection within the slag, with the ongoing passage of oxygen-containing gas through the lance maintaining the lower extent of the lance at a temperature at which the solidified slag coating is maintained and protects the outer pipe.
With a new TSL lance, the relative positions of the lower ends of the outer and inner pipes, that is, the distance the lower end of the inner pipe is set back, if at all, from the lower end of the outer pipe, is an optimum length for a particular pyrometallurgical operating window determined during the design. The optimum length can be different for different uses of TSL technology. Thus, in a two stage batch operation for converting copper matte to blister copper with oxygen transfer through slag to matte, a continuous single stage operation for converting copper matte to blister copper, a process for reduction of a lead containing slag, or a process for the smelting an iron oxide feed material for the production of pig iron, all have different respective optimum mixing chamber length. However, in each case, the length of the mixing chamber progressively falls below the optimum for the pyrometallurgical operation as the lower end of the outer pipe slowly wears and burns back. Similarly, if there is zero offset between the ends of the outer and inner pipes, the lower end of the inner pipe can become exposed to the slag, with it also being worn and subjected to burn back. Thus, at intervals, the lower end of at least the outer pipe needs to be cut to provide a clean edge to which is welded a length of pipe of the appropriate diameter, to re-establish the optimum relative positions of the pipe lower ends to optimize smelting conditions.
The rate at which the lower end of the outer pipe wears and burns back varies with the molten bath pyrometallurgical operation being conducted. Factors which determine that rate include feed processing rate, operating temperature, bath fluidity and chemistry, lance flows rates, etc. In some cases the rate of corrosion wear and burn back is relatively high and can be such that in the worst instance several hours operating time can be lost in a day due to the need to interrupt processing to remove a worn lance from operation and replace it with another, whilst the worn lance taken from service is repaired. Such stoppages may occur several times in a day with each stoppage adding to non-processing time. While TSL technology offers significant benefits, including cost savings, over other technologies, any lost operating time for the replacement of lances carries a significant cost penalty.
With both top blowing and TSL lances, there have been proposals for fluid cooling to protect the lance from the high temperatures encountered in pyrometallurgical processes. Examples of fluid cooled lances for top blowing are disclosed in U.S. patents:                U.S. Pat. No. 3,223,398 to Bertram et al,        U.S. Pat. No. 3,269,829 to Belkin,        U.S. Pat. No. 3,321,139 to De Saint Martin,        U.S. Pat. No. 3,338,570 to Zimmer,        U.S. Pat. No. 3,411,716 to Stephan et al,        U.S. Pat. No. 3,488,044 to Shepherd,        U.S. Pat. No. 3,730,505 to Ramacciotti et al        U.S. Pat. No. 3,802,681 to Pfeifer,        U.S. Pat. No. 3,828,850 to McMinn et al,        U.S. Pat. No. 3,876,190 to Johnstone et al,        U.S. Pat. No. 3,889,933 to Jaquay,        U.S. Pat. No. 4,097,030 to Desaar,        U.S. Pat. No. 4,396,182 to Schaffar et al,        U.S. Pat. No. 4,541,617 to Okane et al; and        U.S. Pat. No. 6,565,800 to Dunne.        
All of these references, with the exception of U.S. Pat. No. 3,223,398 to Bertram et al and U.S. Pat. No. 3,269,829 to Belkin, utilise concentric outermost pipes arranged to enable fluid flow to the outlet tip of the lance along a supply passage and back from the tip along a return passage, although Bertram et al use a variant in which such flow is limited to a nozzle portion of the lance. While Belkin provides cooling water, this passes through outlets along the length of an inner pipe to mix with oxygen supplied along an annular passage between the inner pipe and outer pipe, so as to be injected as steam with the oxygen. Heating and evaporation of the water provides cooling of the lance of Belkin, while stream generated and injected is said to return heat to the bath.
U.S. Pat. No. 3,521,872 to Themelis, U.S. Pat. No. 4,023,676 to Bennett et al and U.S. Pat. No. 4,326,701 to Hayden, Jr. et al purport to disclose lances for submerged injection. The proposal of Themelis is similar to that of U.S. Pat. No. 3,269,829 to Belkin. Each uses a lance cooled by adding water to the gas flow and relying on evaporation into the injected stream, an arrangement which is not the same as cooling the lance with water through heat transfer in a closed system. However, the arrangement of Themelis does not have an inner pipe and the gas and water are supplied along a single pipe in which the water is vaporized. The proposal of Bennett et al, while referred to as a lance, is more akin to a tuyere in that it injects, below the surface of molten ferrous metal, through the peripheral wall of a furnace in which the molten metal is contained. In the proposal of Bennett et al, concentric pipes for injection extend within a ceramic sleeve while cooling water is circulated through pipes encased in the ceramic. In the case of Hayden, Jr. et al, provision for a cooling fluid is made only in an upper extent of the lance, while the lower extent to the submergible outlet end comprises a single pipe encased in a refractory cement.
Limitations of the prior art proposals are highlighted by Themelis. The discussion is in relation to the refining of copper by oxygen injection. While copper has a melting point of about 1085° C., it is pointed out by Themelis that refining is conducted at a superheated temperature of about 1140° C. to 1195° C. At such temperatures lances of the best stainless or alloy steels have very little strength. Thus, even top blowing lances typically utilize circulated fluid cooling or, in the case of the submerged lances of Bennett and Hayden, Jr, et al, a refractory or ceramic coating. The advance of U.S. Pat. No. 3,269,829 to Belkin, and the improvement over Belkin provided by Themelis, is to utilize the powerful cooling able to be achieved by evaporation of water mixed within the injected gas. In each case, evaporation is to be achieved within, and to cool, the lance. The improvement of Themelis over Belkin is in atomization of the coolant water prior to its supply to the lance, avoiding the risks of structural failure of the lance and of an explosion caused by injection of liquid water within the molten metal.
U.S. Pat. No. 6,565,800 to Dunne discloses a solids injection lance for injecting solid particulate material into molten material, using an unreactive carrier. That is, the lance is simply for use in conveying the particulate material into the melt, rather than as a device enabling mixing of materials and combustion. The lance has a central core tube through which the particulate material is blown and, in direct thermal contact with the outer surface of the core tube, a double-walled jacket through which coolant such as water is able to be circulated. The jacket extends along a part of the length of the core tube to leave a projecting length of the core tube at the outlet end of the lance. The lance has a length of at least 1.5 meters and from the realistic drawings, it is apparent that the outside diameter of the jacket is of the order of about 12 cm, with the internal diameter of the core tube of the order of about 4 cm. The jacket comprises successive lengths welded together, with the main lengths of steel and the end section nearer to the outlet end of the lance being of copper or a copper alloy. The projecting outlet end of the inner pipe is of stainless steel which, to facilitate replacement, is connected to the main length of the inner pipe by a screw thread engagement.
The lance of U.S. Pat. No. 6,565,800 to Dunne is said to be suitable for use in the Hlsmelt process for production of molten ferrous metal, with the lance enabling the injection of iron oxide feed material and carbonaceous reductant. In this context, the lance is exposed to hostile conditions, including operating temperatures of the order of 1400° C. However, as indicated above with reference to Themelis, copper has a melting point of about 1085° C. and even at temperatures of about 1140° C. to 1195° C., stainless steels have very little strength. Perhaps the proposal of Dunne is suitable for use in the context of the Hlsmelt process, given the high ratio of about 8:1 in cooling jacket cross-section to the cross-section of the core tube, and the small overall cross-sections involved. The lance of Dunne is not a TSL lance, nor is it suitable for use in TSL technology.
Examples of lances for use in pyrometallurgical processes based on TSL technology are provided by U.S. Pat. Nos. 4,251,271 and 5,251,879, both to Floyd and U.S. Pat. No. 5,308,043 to Floyd et al. As detailed above, slag initially is splashed by using the lance for top blowing top blowing onto a molten slag layer to achieve a protective coating of slag on the lance which is solidified by high velocity top blown gas which generates the splashing. The solid slag coating is maintained despite the lance then being lowered to submerge the lower outlet end in the slag layer to enable the required top submerged lancing injection within the slag. The lances of U.S. Pat. Nos. 4,251,271 and 5,251,879, both to Floyd, operate in this way with the cooling to maintain the solid slag layer being solely by injected gas in the case of U.S. Pat. No. 4,251,271 and by that gas plus gas blown through a shroud pipe in the case of U.S. Pat. No. 5,251,879. However, with U.S. Pat. No. 5,308,043 to Floyd et al cooling, additional to that provided by injected gas and gas blown through a shroud pipe, is provided by cooling fluid circulated through annular passages defined by the outer three pipes of the lance. This is made possible by provision of an annular tip of solid alloy steel which, at the outlet end of the lance, joins the outermost and innermost of those three pipes around the circumference of the lance. The annular tip is cooled by injected gas and also by coolant fluid which flows across an upper end face of the tip. The solid form of the annular tip, and its manufacture from an alloy steel, result in the tip having a good level of resistance to wear and burn back. The arrangement is such that a practical operating life is able to be achieved with the lance before it is necessary to replace the tip in order to safeguard against a risk of failure of the lance enabling cooling fluid to discharge within the molten bath.
The present invention relates to an improved fluid cooled, top submerged injecting lance for use in TSL operations. The lance of the present invention provides an alternative choice to the lance of U.S. Pat. No. 5,308,043 to Floyd et al but, at least in preferred forms, can provide benefits over the lance of that patent.